For a number of years the coating industry has been engaged in substantial developmental programs in the quest for procedures which would allow the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed commensurate with the contiguous processing steps. The industry has desired to eliminate the volatile solvents required in many of the well-known coating processes because of potential hazards or because of the cost of equipment to handle the evolved solvent vapors. In addition, the industry has been seeking coating formulations which would produce coatings which were durable and which would permit substantial additional processing of the workpiece, such as metal forming operations where the substrate is metal strip for container bodies, blanks and closures, or where plastic strip and paperboard are coated and formed.
Epoxy coating formulations have long been recognized as affording desirable properties in the finished coating, especially the toughness to withstand further processing. However, the problem has remained to develop a low cost epoxy coating formulation which would combine the desired rheological properties for the coating application with both reasonable pot life and rapid curing in the production line.
In Schlesinger U.S. Pat. No. 3,708,296 granted Jan. 2, 1973, there are disclosed photopolymerizable epoxide formulations containing diazonium salts as photoinitiators which polymerize rapidly upon exposure to electromagnetic radiation to provide durable coatings.
In Watt U.S. Pat. No. 3,794,576 granted Feb. 26, 1974, there are described desirable epoxy formulations which combine the desired rheological properties with suitable pot life and rapid curing at ambient temperatures, by incorporation of a photoinitiator and at least about 15 percent by weight of an epoxidic ester having two epoxycycloalkyl groups.
Since the disclosures of Schlesinger and Watt, a number of patents and publications have appeared proposing various onium salt photoinitiators for the epoxy formulations which could replace the diazonium catalysts specifically described in the Schlesinger and Watt Patents. Among these are the sulfonium and other Group VIa salt catalysts disclosed in Smith U.S. Pat. No. 4,069,054 granted Jan. 17, 1978; Barton U.S. Pat. No. 4,090,936 granted May 23, 1978; and Crivello U.S. Pat. Nos. 4,069,055 granted Jan. 17, 1978 and 4,058,401 granted Nov. 15, 1977. Other onium salt catalysts are described in Crivello U.S. Pat. Nos. 4,069,054 granted Jan. 17, 1978 and 3,981,897 granted Sept. 21, 1976; and UV CURING: SCIENCE AND TECHNOLOGY, edited by S. P. Pappas et al (Technology Marketing Corporation, Stamford, Conn.).
Use of sulfonium catalysts in combination with epoxide formulations containing the epoxycycloalkyl esters of the aforementioned Watt Patent is now being advocated commercially by various companies to provide systems with good storage and ambient temperature curing characteristics. More recently, it has been suggested that epoxide formulations which do not contain the relatively expensive epoxycycloalkyl esters of the Watt patent may be cured rapidly upon exposure to electromagnetic radiation by maintenance of the epoxide formulation at a controlled elevated temperature for a short period of time after exposure.
However, there has continued a search for other epoxide compounds which would provide the highly desirable rapid, ambient temperature curing characteristics to photopolymerizable epoxide compositions to provide durable commercial coatings. As will be appreciated, such compositions must exhibit shelf stability, acceptable rheological properties, and good physical properties for the finished coating as well as rapid ambient temperature curing properties. Moreover, desirably the monomer cost should be relatively economical.
Accordingly, it is an object of the present invention to provide novel photopolymerizable compositions containing epoxidic ether compounds which may be cured at ambient temperatures.
It is also an object to provide such compositions which are relatively stable during extended periods of storage and which will nevertheless polymerize rapidly to produce coatings with desirable mechanical properties.
Another object is to provide a novel polymerization process using such compositions and which is adaptable to a wide variety of high speed coating lines and which does not require extensive or expensive equipment to effect polymerization of the applied coating.